[SOLVED] Why HFSS Eigenmode cannot recognize my resonator

[Zluo]

Hi,

I'm trying to design a microwave filter with coaxial-SIW structure. Basically, it's a coaxial line with shorted one side surrounding by SIW. The length of the line is less than 1/8 lamda that's why it can be represented by an inductor. On top side, there is a big lumped capacitor, see picture below. That's how a resonator was buildup. However, when I try to use HFSS eigenmode to analyze the design, I can only find SIW waveguide mode but no TEM coaxial mode can be recognized.

Then I turn back to HFSS modal and do spectroscopy. From S21, see below, i can see the resonance clearly located at around 7.3GHz with coaxial mode. However, the smallest resonance frequency by Eigenmode is around 15GHz with waveguide mode.

Could some one help me on this? Do you have similar problem with eigenmode?

 

[PlanarMetamaterials]

Welcome Zluo,

I assume you have the minimum eigenmode frequency set to something reasonable (e.g., 5 GHz or lower).

I suspect that either the TEM mode is too strongly coupled to a waveguide mode, and hence being obscured by the solver; or more likely, there is some error in the simulation boundary conditions. How are you dealing with the feed in the eigenmode solution?

 

[Zluo]

Welcome Zluo,

I assume you have the minimum eigenmode frequency set to something reasonable (e.g., 5 GHz or lower).

I suspect that either the TEM mode is too strongly coupled to a waveguide mode, and hence being obscured by the solver; or more likely, there is some error in the simulation boundary conditions. How are you dealing with the feed in the eigenmode solution?

Thank you for your reply.

Yes, it's true. I set min. eigenmode frequency 3GHz..
And I have the same suspicions as you mentioned. I don't have any boundaries defined. Maybe I should add a vacuum box to surround the model. I don't define any port or excitations in eigenmode since it is not necessary based on the 'Help' documents.

 

[PlanarMetamaterials]

I don't define any port or excitations in eigenmode since it is not necessary based on the 'Help' documents.

Yes, that's right, there are no excitations in an eigenmode solution.

Maybe I should add a vacuum box to surround the model.

This is most likely your issue, yes. In the absence of boundaries, HFSS will insert PECs along the bounding edges of your model. This would effectively short out your feed line and prevent the TEM mode from being solved.

Simulating such boundaries in HFSS' eigenmode solver is a bit of a trick. You cannot do a vacuum box, since the domain needs to be closed. I might suggest including an impedance boundary overtop of the feed region, or including a short section of coaxial cable, and then terminating this in an impedance boundary. The rest can probably be PECs, since that's what you effectively have anyways.